These studies demonstrate the value of developing microbiological criteria for use in periodontal diagnosis and for evaluating the efficacy of therapeutic measures. Phase-contract microscopy was employed to evaluate the subgingival microflora at chairside in persons with adult periodontitis, juvenile periodontitis, marginal gingivitis and excellent periodontal health. Microbial risk factors for periodontal disease were recognized and a Microbial Index for scoring phase-contrast microscopic observations was formulated. Antimicrobial therapeutic methods were evaluated (NaHCO3, NaC1, MgSO4 and tetracycline) in a blind fashion for use in augmenting the antimicrobial effects of scaling and root planing in periodontal treatment. Various patient groups were treated with antimicrobial therapy of scaling, root planing, subgingival application of antimicrobial agents, and systemic administration of antibiotics in the presence of advanced lesions. All groups had dramatic reductions in pocket depth and increases in attachment levels following the elimination of microbial risk factors. Teeth with hopeless prognoses under conventional standards stabilized in most cases with clinical improvements.